Conventionally, a so-called strobe apparatus which instantaneously illuminates an object to be photographed has been widely used in cameras for taking silver halide pictures and in digital cameras for sensing images by using image sensing devices.
As an example of this strobe apparatus, the present applicant has proposed a strobe apparatus shown in FIG. 6 in Japanese Patent Application No. 2000-395929 and Japanese Patent Application No. 2001-160983 (note, the Japanese Patent Applications are not laid-opened when the present application is filed) including claim of its internal priority, in order to realize a space-saving strobe apparatus with a simple circuit configuration.
That is, this strobe apparatus shown in FIG. 6 roughly comprises a discharge tube (e.g., a xenon discharge tube) 4 for flash, a discharge capacitor 3 which stores electrical energy for allowing the discharge tube 4 to emit light, a charging circuit 1 for charging electrical energy in the discharge capacitor 3, and a trigger circuit 2 which generates a high-voltage signal for causing the discharge tube 4 to discharge.
This circuit has a boosting circuit (11, 12, and 13) which includes a piezoelectric transformer 13 and is shared as a boosting means by the charging circuit 1 and trigger circuit 2. A switch (or switching element) 14 is inserted in series with a line connecting the output of the piezoelectric transformer 13 and the discharge capacitor 3, in order to apply the output voltage from the piezoelectric transformer 13 to the discharge capacitor 3 or discharge tube 4. By properly turning on and off this switch 14, charging of the discharge capacitor 3 and triggering of the discharge tube 4 are realized.
With the above circuit configuration, both the charging function and triggering function can be achieved by only one boosting circuit. Accordingly, a space-saving strove apparatus can be realized.
In the above circuit configuration, however, the high AC voltage (about a few kV) output from the piezoelectric transformer 13 is applied to the switch 14 in accordance with its switching operation. Therefore, it is necessary to use a high-withstand-voltage switching element capable of withstanding this high AC voltage.
Unfortunately, a high-withstand-voltage switching element which can be used, when the present application is filed, as the switch 14 to turn on and off the output voltage of the piezoelectric transformer is expensive and has a relatively large size. This interferes with miniaturization of the strobe apparatus as a whole. To meet demands for further miniaturization of the strobe apparatus, the circuit configuration shown in FIG. 6 is still unsatisfactory.
International Publication (domestic re-publication of PCT international publication) No. W097/29521 has proposed a plurality of different piezoelectric transformers driven by a one-wave resonance mode, for the purpose of obtaining an output equivalent to those obtained by the conventional piezoelectric transformers by using a piezoelectric transformer smaller than those conventional piezoelectric transformers at an oscillation rate lower than the natural oscillation level limit of the material.
FIG. 7 is a perspective view (corresponding to FIG. 6 of International Publication No. WO97/29521) showing the piezoelectric transformer proposed in International Publication No. WO97/29521. The arrow indicates the polarization direction.
In this publication, a piezoelectric transformer 121 has a primary-side region which occupies ½ in the longitudinal direction of a rectangular plate 123 as a single-plate piezoelectric element. In this primary-side region, surface electrodes 124 and 125 as input electrodes are formed. In a secondary-side region, strip electrodes 126 and 127 as output electrodes are formed in a position which is ¼ in the longitudinal direction of the rectangular plate 123 from the end portion of the region. In addition, an end-face electrode 132 as an output ground electrode is formed at the end face of this secondary-side region.
In this publication, before the piezoelectric transformer 121 having the structure shown in FIG. 7 is driven, one of the surface electrodes 124 or 125, and the end-face electrode 132 are grounded. Consequently, an AC voltage having a frequency whose wavelength is equal to the length of the rectangular plate 123 is applied between the surface electrodes 124 and 125, thereby driving the piezoelectric transformer 121 in a one-wave resonance mode (λ mode)
In this piezoelectric transformer 121 driven in the one-wave resonance mode, an output voltage appears in the strip electrodes 126 and 127. This output voltage is extracted from between at least one of the strip electrodes 126 and/or 127, and the grounded one of the surface electrodes 124 or 125, or between at least one of the strip electrodes 126 and/or 127, and the grounded end-face electrode 132. As described in the publication, the connection when this piezoelectric transformer 121 is in operation is used to decrease the oscillation rate and generated heat and increase the efficiency compared to the conventional piezoelectric transformers, as indicated by the above-mentioned purpose.